Can a GSM iPhone 5s be activated through iTunes?
With GSM phones and their evolutions, you will need a SIM (Subscriber Information Module) card. The SIM card is what is activated to its' network, not the phone.
Originally Posted by iMore Question
CDMA 2000 network phones used in the US and North America, generally don't use a SIM card, because your subscriber info is kept on the cell phone itself and the phone itself is activated to use the network.
It's like this analogy: with CDMA 2000, when you get to the door of the party, the bouncer looks to see if you are on the list. If you are on the list, you are let in. With GSM, your SIM card is your invitation. You show your SIM card to the bouncer and he lets you in.